1. Field of the Invention
The present invention relates generally to systems and methods for noise attenuation, and more particularly to a method and apparatus for attenuating noise through phased accumulation and confinement of compressible flow mass and noise, whereby noise is attenuated by ringdown. The noise attenuation method of the present invention has utility both in exhaust systems and intake systems, and has particular utility in exhaust systems of engines operable under elevated back pressure conditions.
2. Related Art
Sound, including sound noise, is generated by pressure fluctuation in a medium, where the pressure fluctuation propagates through the medium in the form of a pressure wave; the pressure wave transmits acoustic energy. The medium may be solid or fluid, such as liquid, gas or a mixture thereof.
Conventional noise attenuation systems and methods utilize basic sound propagation and dissipation principles to attenuate noise generated by a source, such as the exhaust noise of an engine. Generally, such noise attenuation systems and methods may be characterized as active type or passive type.
Active type noise attenuation systems and methods include noise cancellation pressure waves generated using various electromechanical feed-forward or feed-back control elements and techniques. For example, a source of cancellation sound may be provided in communication with a source of undesirable noise and controlled so as to generate sound/pressure wave fluctuations that are complimentary to the sound/pressure wave fluctuations of the undesirable noise, where the complimentary sound and undesirable noise pressure wave fluctuations are superimposed on each other such that the respective pressure wave fluctuations cancel each other out.
Passive type noise attenuation systems and methods are those whose noise attenuation performance is a function of the geometry and sound absorbing properties of the system components. Sound, that is, acoustic energy transmitted in the form of pressure waves, decays naturally by conversion into heat. This conversion may occur by either one or both of i) molecular relaxation in the bulk of the acoustic propagation medium, and ii) interaction between the pressure wave/medium and any sound absorbing boundaries of the system, such as sound absorbing walls, linings, and the like.
Conventional active type and passive type systems may include one or any number of noise attenuating components or elements, such as pipes, chambers, ducts, reflection walls, projections, perforated structures, and the like, or portions thereof, lined or unlined, variously arranged to provide area discontinuities, impedance discontinuities, reflective surfaces, absorptive surfaces, and the like, for directing, reflecting, absorbing and attenuating noise (acoustic energy/pressure waves).
A discussion of various conventional noise attenuation structures, their operating principles, and various analytical methods, including the transfer matrix approach and the finite-element, boundary element, and acoustical-wave finite-element methods, may be found in Beranek and Ver, xe2x80x9cNoise and Vibration Control Engineering; Principles and Applicationsxe2x80x9d, John Wiley and Sons, Inc. (1992).
FIG. 1 schematically illustrates a generic silencer (muffler) 110 utilizing conventional passive type noise attenuating elements and methods. As shown therein, exhaust (a compressible flow mass) and noise from a noise source (shown in phantom) 112, such as an engine, flow through a transmittance path including an inlet 114, a plurality of passive type noise attenuating elements (e.g., tubes, chambers, perforated structures, and the like), and an outlet 116 to an external environment (shown in phantom) 118. As shown by arrows therein, noise (acoustic energy/pressure waves) from the noise source generally is directed and redirected at impedance discontinuities, walls and other structural features, so as to be attenuated. By design, conventional noise attenuation systems such as the silencer of FIG. 1 feature a continuously open transmittance path for flow of compressible exhaust mass and noise, between the noise source and the external environment. Noise attenuation is achieved through (1) acoustic wave reflection at cross-sectional discontinuities, which impede sound propagation but permit a continuous gross flow of compressible exhaust mass, and (2) acoustic energy dissipation resulting from sound wave interaction with absorptive boundaries or walls. As schematically illustrated in FIG. 1, for example, an acoustic wave (noise) incident at inlet 114 of silencer 110 (see large arrow 115A) is attenuated as it flows through and exits silencer 110 (see small arrow 115B). Attenuation of the acoustic wave is achieved by reflections at impededence discontinuities (see, e.g., arrow 115C) and by absorption, e.g., at absorptive boundary 119 (see successively diminishing arrows 115D, 115E, 115F). Conventional noise attenuation systems have a relatively steady (substantially continuous) gross flow of compressible exhaust mass through a defined transmittance path, where the gross flow experiences superimposed fluctuations at the source (source volume flow cycle) under fixed operating conditions, such as an engine exhaust cycle.
Although conventional noise attenuation systems and methods have utility in many applications, such systems and methods have a drawback in that achievable noise attenuation is limited because the interaction of the propagating sound waves with noise attenuating structures in such conventional systems generally is limited to the time the sound waves (noise) take to propagate through the length of the transmittance path of the noise attenuating system. A need exists for an improved system and method for attenuating noise.
The present invention generally provides a novel method for phased noise attenuation. According to the present invention, a noise attenuation method is provided in which flow of compressible flow mass and noise is phased, by periodically accumulating and substantially confining a compressible fluid flow mass and noise in at least one defined volume of a transmittance path, for a time sufficient to attenuate noise confined in the defined volume by ringdown. A noise attenuation system using a method of the present invention thus provides a non-continuous transmittance path for compressible flow mass and noise between a noise source and an external environment.
The present invention also generally relates to a noise attenuation system and method which provides periodic physical blockage of a compressible fluid borne sound transmission path, such as from an engine to an external environment, at a plurality of different locations of the transmittance path, providing temporary confinement of compressible flow mass and acoustic energy in at least one of plural accumulators of the system (noise attenuator), whereby noise is attenuated by a prolonged period of a) sound absorption by propagation in the medium within the accumulator, and/or b) sound interaction with dissipative accumulator surfaces and boundaries, and wherein the overall transmittance path is continuously, selectively blocked (xe2x80x9cnon-continuousxe2x80x9d).
In one aspect, the present invention relates to a noise attenuator including a transmittance path for compressible flow mass and noise, and phased transmittance means for selectively accumulating and confining compressible flow mass and noise in a defined volume of the transmittance path so as to attenuate noise within the defined volume by ringdown.
In another aspect, the present invention relates to a noise attenuator including a plurality of accumulators arranged in series and collectively providing a transmittance path for compressible flow mass and noise between a noise source and an external environment, where the plurality of accumulators include a first accumulator disposed immediately adjacent the noise source, and phased transmittance means for selectively accumulating and confining compressible flow mass and noise from the noise source within at least one of the plurality of accumulators disposed downstream of the first accumulator, at respective timings, thereby attenuating exhaust noise within the at least one accumulator by ringdown.
In yet another aspect, the present invention relates to a noise attenuator including a plurality of accumulators each providing an independent transmittance path for compressible flow mass and noise between a noise source and an external environment, and phased transmittance means for selectively accumulating and confining compressible flow mass and noise from the noise source within each of the plurality of accumulators, at respective timings, thereby attenuating noise within each of the plurality of accumulators by ringdown.
In still another aspect, the present invention relates to a noise attenuator including a first accumulator and a second accumulator providing a transmittance path for compressible flow mass and noise between a noise source and an external environment, a first interrupter provided in fluid communication with the first accumulator and selectively operable to effectively block the transmittance path through the first accumulator at a first timing, and a second interruptor provided in fluid communication with the second accumulator and selectively operable to effectively block the transmittance path through the second accumulator at a second timing, different than the first timing, whereby the transmittance path from the noise source to the external environment is non-continuous.
In one embodiment, the first accumulator and the second accumulator are arranged in series configuration.
In another embodiment, the first accumulator and the second accumulator are arranged in parallel configuration.
In each embodiment, each interruptor may be a conventional valve, such as a pipe valve, controlled by conventional control elements well known in the art.
In another aspect, the present invention relates to a method for attenuating noise in a noise transmittance path, including selectively, e.g., periodically, accumulating and confining compressible flow mass and noise within at least one defined volume of the transmittance path and attenuating the noise confined within the defined volume by ringdown.
In one embodiment, the method includes forming the noise transmittance path using a plurality of accumulators, and periodically confining the compressible flow mass and noise in the defined volume using a plurality of interrupters.
In another aspect, a noise attenuation method and system according to the present invention may be used with a conventional engine, whereby exhaust and noise from the engine are selectively accumulated and confined in at least one accumulator, for a time sufficient to attenuate the exhaust noise by ringdown.
These and other objects, features and advantages will be apparent from the following description of the preferred embodiments of the present invention.